1. Field of the Invention
The present invention relates to a rectifier station for a DC transmission which includes a rectifier bridge with a control system.
More particularly, the invention relates to protecting devices for overvoltages which may occur when the rectifier is started and controlled towards a so-called "open" line, that is, a line that is broken so that it does not conduct any current, either due to a fault on the line itself, or which is more common, due to a fault in the inverter station connected to the line. Since no current flows in the line, the rectifier control will control the rectifiers towards the smallest delay angle and thus the maximum voltage. As a consequence of a failing line current, a so-called top rectification will occur, that is, charging of the capacitances of the line to a voltage corresponding to the amplitude value of the alternating voltage without the voltage smoothing which a current would lead to. To this is to be added the voltage reflection on the line, which is an inevitable result of the open line. In particular in the case of cable transmissions which are very sensitive to overvoltages and also have great capacitances, such overvoltages place heavy demands on the discharge protection of the line.
2. The Prior Art
Instead of waiting for the discharge protectors to start operating, various solutions have been proposed to indicate overvoltages in time and to reduce the current of the rectifier, or at least limit its current increase.